1. Field of the Invention
The present invention relates generally to superconducting cables experiencing reduced strain, and more specifically to such superconducting cables that are composed of A-15 materials.
2. Discussion of the Related Art
Superconductors are phases that exhibit extremely low (practically zero) electrical resistance below their critical temperature and critical magnetic field. Superconducting cables have been used in a variety of applications, predominantly in superconducting electromagnetic magnets in which a superconductor is wound into a coil. Superconducting magnets have been used in applications including, for example, devices used for nuclear magnetic resonance (NMR) spectroscopy, magnetic resonance imaging (MRI), superconducting magnetic energy storage (SMES) and magnetic mine sweeping, as disclosed in, for example, Superconducting Magnets, M. N. Wilson, Oxford University Press, New York, N.Y. (1983) (hereinafter xe2x80x9cWilsonxe2x80x9d) and Case Studies in Superconducting Magnets, Y. Iwasa, Plenum Press, New York, N.Y. (1994) (hereinafter xe2x80x9cIwasaxe2x80x9d).
To wind a coil, of course, the material defining the coil must be bent. The smaller the coil, the more the material defining the coil must be bent. Since superconducting magnets in many cases are made of a relatively small coil, the superconducting material defining the coil must be bent significantly. Even in the case of relatively large coils, bending superconductors to make coils according to prior art methods can be problematic due to the relatively large cross-sectional size of the superconductors typically used in these applications. One reason that superconducting magnets might desirably be small is in NMR applications where the intensity of the magnetic field is critical. Stored energy of the magnet system and its overall cost scale directly with the size of the bore of the superconducting magnet where intense magnetic fields are produced. In general, for two superconducting magnet systems designed and fabricated to produce a given magnetic field strength, the system with a smaller superconducting magnet bore will be less costly to fabricate and operate. In order to wind a superconductor around a magnet bore, the superconductor must be bent significantly. The lower limit on the radius of curvature to which a superconductor, such as in a superconducting cable, can be wound within a magnet system, such as an NMR, MRI, or other practical magnet systems, is usually determined by the irreversible strain (defined below) of the superconductor.
Superconducting wires are typically comprised of a plurality of superconducting filaments disposed within a matrix that is typically formed of an electrically conducting material, such as metals and metal alloys. Typically, superconducting cables are formed of a plurality of intertwined wires including superconducting wires. Superconducting cables are often used for large current applications and may include additional wires that are not superconducting in order to provide physical support to the cable and/or to act as a current stabilizing medium should superconductivity of any of the superconducting wires be interrupted.
When a superconductor or superconducting cable is bent, strain is induced on the superconducting filaments. Since many superconductors are brittle, bending them can cause them to break. That is, in winding superconducting coils, if the strain surpasses the irreversible strain of the material from which the superconducting filaments of the cable are formed, the potential magnetic field of the system can not be achieved. Hence, for a given superconductor or superconducting cable, there is a lower limit on the radius of curvature to which the superconductor or superconducting cable can be wound within the magnet system, dependant on the irreversible strain of the superconducting filaments within the superconductor or superconducting cable.
Known superconductors must be cooled to be made superconducting and must be kept cool to remain superconducting, for example, in a bath of liquid helium. The intensity of the magnetic field produced by a superconducting magnetic generally scales with the number of turns of the superconductor or superconducting cable present. Generally, a superconductor, such as superconducting wire or cable, is wound around a support structure or coil form a number of times in order to produce a desired magnetic field. In order to eliminate undesirable electrical current flow between the windings and/or turns, superconductors are advantageously electrically insulated. In conventional systems involving brittle superconductors, electrical insulation of the superconductors, such as superconducting wires or cables, is typically performed after the superconductor is wound around the support or coil form. This method is limited in its efficiency because it cannot always optimize the ratio of conductor to non-conductor present in the windings.
A15 superconductors are known intermetallic compounds (defined below) that have relatively high critical temperatures and critical magnetic fields compared to other conventional superconducting alloys, so it is desirable to employ A15 superconductors in many magnet systems, particularly such systems that are designed for use with magnetic fields of above 10 Tesla, typically from about ten to about 24 Tesla. While A15 superconductors can provide certain superior performance characteristics, these are inherently brittle and have relatively low irreversible strains. Therefore, monolithic A15 superconductors (those which comprise a continuous medium or whose members are bonded together) typically cannot be wound to a small enough radius of curvature to be useful for winding into coils in fabricating many magnet systems. In an attempt to overcome this problem, a xe2x80x9cwind-then-reactxe2x80x9d (or xe2x80x9cwind-and-reactxe2x80x9d) method has been commonly used to incorporate A15 superconductors into magnet systems. As described in Wilson and Iwasa, the xe2x80x9cwind-then-reactxe2x80x9d method involves winding unreacted cables around a support or coil form and subsequently heating the entire magnet system to cause a reaction within the unreacted cables to form superconducting filaments (filaments comprising a superconducting phase) within the cables. However, this approach has several disadvantages in many cases. For example, since heating occurs after the cable is wound within the magnet system, the various components of the magnet system should be compatible with the temperatures used during the formation of the superconducting phase (e.g., about 925 K for Nb3Sn). This can severely limit the choice of materials from which various components of the magnet system can be formed. For example, the magnet system often cannot include aluminum or its alloys since these melt at the temperatures used during formation of the superconducting filaments. Another important disadvantage is the difficulty and expense of applying insulation to a magnet winding to effectively coat the individual conducting wires or cables to prevent electrical current flow between the windings/turns. Hence, the xe2x80x9cwind-then-reactxe2x80x9d method can result in higher cost and complexity in preparing the magnet system, while resulting in a system that may offer inferior performance.
An alternative to the xe2x80x9cwind-then-reactxe2x80x9d method is the xe2x80x9creact-then-windxe2x80x9d technique. As discussed in Wilson and Iwasa, the xe2x80x9creact-then-windxe2x80x9d method involves forming the superconducting filaments within superconducting cables by heat reacting and subsequently winding the cables into the magnet system. Since heating of the cables occurs prior to their incorporation into the magnet system, the xe2x80x9creact-then-windxe2x80x9d method allows for a broader range of materials from which the components of the magnet system can be formed. However, the low irreversible strain of many superconductors has precluded the broad use of the xe2x80x9creact-then-windxe2x80x9d method or systems with small bores with these superconductors. Instead, the xe2x80x9creact-then-windxe2x80x9d method has typically been confined to systems such as superconductors having tape-like cross-sections (where the typical ratio of width to thickness is larger than 10). M. J. Leupold Cryogenics 24, 1413-1417 (1988) (hereinafter xe2x80x9cLeupoldxe2x80x9d) and B. Jakob et al., IEEE Trans. on Magn. 24, 1437-1439 (1988) (hereinafter xe2x80x9cJakobxe2x80x9d) disclose typical tape-like A15 superconductors appropriate for use with the xe2x80x9creact-then-windxe2x80x9d method. Despite their ability to be incorporated into magnet systems using the xe2x80x9creact-then-windxe2x80x9d method, magnet systems having tape-like superconductors have several disadvantages relative to A15 superconducting cables. For example, tape-like A15 superconductors are typically monolithic and are design/application specific, and systems employing these superconductors can not be readily scaled to allow for changes in current-carrying capacity.
While the above and other documents describe, in many cases, useful superconducting arrangments, including high temperature superconductors, there exists a for, and applications of, improved superconductors, including superconducting cables and systems incorporating brittle superconductors, appropriate for use in many of the common applications for superconductors, such as in superconducting magnet systems. It is an object of the invention to provide improved superconducting cables, magnet systems, and methods for their production and use.
In one embodiment, the present invention provides a cable which comprises a plurality of wires. At least one of the wires, alternatively a plurality of the wires, has at least one filament formed of a brittle superconductor. The wires are intertwined to define intimate contact surface areas at least 50% of which are free of inter-wire bonds.
In another embodiment, the present invention provides a cable which comprises a plurality of wires. At least one of the wires has at least one filament formed of a brittle superconductor. The wires are intertwined so that a strain on at least one filament due to bending of the cable is essentially independent of a cross-sectional radius of the cable.
In yet another embodiment, the present invention provides a superconducting cable which comprises a plurality of wires with at least one of the wires having at least one filament formed of a brittle superconductor. At least one of the superconducting wires having at least one filament formed of a brittle superconductor undergoes a degradation in critical current density of at least about 10% beyond an axial strain of about 2% and has a cross-sectional dimension of at most about 0.5 millimeter.
In a still another embodiment, the present invention provides a method of making a magnet system. The method comprises the steps of providing a cable and forming the cable into a coil. The cable is formed of a plurality of intertwined wires. At least one of the wires is a superconducting wire having at least one filament formed of a brittle superconductor. At least one superconducting wire having at least one filament formed of a brittle superconductor undergoes a degradation in critical current density of at least 10% beyond an axial strain of about 2%. When formed into a coil of minimum radius of curvature less than about 0.25 meter, the cable retains a cable critical current density of at least about 90%.
In a another embodiment, the present invention provides a method of making a magnet system. The method comprises the steps of providing a cable and forming the cable into a coil. The cable is formed of a plurality of intertwined wires. At least one of the wires is a superconducting wire having at least one filament formed of a brittle superconductor. At least one superconducting wire having at least one filament formed of a brittle superconductor undergoes a degradation in critical current density of at least 10% beyond an axial strain of about 2% and has a maximum cross-sectional dimension of about 0.5 millimeter.
In a further embodiment, the present invention provides a method of making a magnet system. The method includes the steps of providing a cable and forming the cable into a coil. The cable is formed of a plurality of intertwined wires. At least one wire has at least one filament formed of a brittle superconductor. The intertwined wires define intimate contact surface areas at least 50% of which are free of inter-wire bonds.
In yet a further embodiment, the present invention provides a method of making a magnet system. The method includes the steps of providing a cable and forming the cable into a coil. The cable is formed of a plurality of wires. At least one of the wires has at least one filament formed of a brittle superconductor. The wires are intertwined so that the strain experienced by at least one filament due to bending of the cable is essentially independent of a cross-sectional radius of the cable.
In another aspect, the present invention provides a method for making a magnet system that is able to produce a magnetic field of at least 10 Tesla. The method involves winding a cable around a support structure having a minimum radius of curvature less than about 0.25 m. The cable is formed of a plurality of wires at least one of which has at least one filament formed of a brittle superconductor.
In still a further embodiment, the present invention provides a magnet system which comprises a cable wound around a support structure. The cable is formed of a plurality of wires. At least one of the wires has at least one filament formed of a brittle superconductor. The wires are intertwined so that a strain on at least one filament due to bending of the cable is essentially independent of a cross-sectional radius of the cable.
In another embodiment, the present invention provides a magnet system which comprises a cable wound around a support structure. The cable is formed of a plurality of wires. At least one wire has at least one filament formed of a brittle superconductor. The wires are intertwined to define intimate contact surface areas at least 50% of which are free of inter-wire bonds.
Also provided by the invention are methods for producing a superconducting cable. In one aspect, the invention involves a method of making a superconducting cable. The method comprises intertwining a plurality of wires, at least one of which contains at least one superconducting filament, so that more than 50% of the intimate contact surface areas are free of inter-wire bonds. In another embodiment, the method comprises intertwining a plurality of wires and subsequently reacting the wires to form at least one brittle, superconducting filament while leaving more than 50% of the intimate contact surface areas free of inter-wire bonds.
In yet another aspect, the invention provides a method of making a superconducting cable from a plurality of wires, at least one of which contains at least one filament formed from a brittle superconductor. The method involves intertwining the wires to form a cable so that the strain on the wires within the cable due to bending of the cable is essentially independent of a cable cross-sectional radius. In another embodiment, the method comprises intertwining a plurality of wires to form a cable and subsequently reacting the wires to form at least one brittle, superconducting filament so that the strain on the wires within the cable is essentially independent of a cable cross-sectional radius.
In another embodiment, the invention provides a method of fabricating a superconducting cable by intertwining a plurality of wires with at least one wire having at least one filament formed of a brittle superconductor. At least one of the superconducting wires having at least one filament formed of a brittle superconductor undergoes a degradation in critical current density of at least 10% beyond an axial strain of about 2% and has a maximum cross-sectional dimension of 0.5 mm. In another embodiment, the method comprises intertwining a plurality of wires to form a cable and subsequently reacting the wires to form at least one brittle, superconducting filament, where at least one wire having at least one filament formed of a brittle superconductor undergoes a degradation in critical current density of at least 10% beyond an axial strain of about 2% and has a maximum cross-sectional dimension of about 0.5 mm.
Other advantages, novel features, and objects of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings, which are schematic and which are not intended to be drawn to scale. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a single numeral. For purposes of clarity, not every component is labeled in every figure.